Conventional communication systems have relied upon a single antenna transmitter and single antenna receiver system. However, such systems have a capacity which is fundamentally limited for a given bandwidth and signal-to-noise ratio of the received signal. The use of multiple antennas at the receiver and transmitter (MIMO) extends the system into the spatial domain and fundamentally increases its capacity. A system may then be designed which has a larger potential throughput. A system which has equal numbers of transmit and receive antennas has an inherent capacity which scales approximately linearly with the number of antennas. This is achieved by reuse of the same temporal/spectral channel using independent spatial propagation modes that may be separated at the receiver through signal processing. Consequently such radio systems are becoming increasingly important.
However, the linear relationship between antenna number and fundamental capacity is an over-simplification. In practice, to realise this potential capacity, the receiver has the task of separating the interfering spatial sub-channels. This requires the receiver to determine a sufficient number of independent equations which may be solved to isolate the spatial sub-channels. With current systems, this requires that several unique spatial propagation modes, known as eigenmodes, exist which connect the transmitter and receiver. If the number of strong eigenmodes is lower than the number of transmit (or receive elements) then the potential capacity of the system is reduced.
A channel contains a large number of spatial eigenmodes when there are a large number of rays with a large angular separation that connect the transmitter and the receiver. This situation occurs when a number of scatterers, i.e. objects causing scattering of the transmitted signal, are located around the antennas. FIG. 1 shows how the performance of an uncoded BLAST (Bell Labs Layered Space-Time) architecture, with 6×8 MIMO channel, is degraded (bit error rate (BER) increases) when the angular diversity is small. These results were obtained using a wide sense stationary uncorrelated scattering (WSSUS) channel model. An aim of the present invention is to reduce the performance difference between these two extremes.
Digital beamforming can be used to increase the spatial separation of signals (or to increase the angular acceptance of the receive array). However, this system requires complex digital processing electronics for conditioning the signals prior to feeding to the antenna array for transmission.
With current MIMO systems, there is a danger than the potential throughput may not be achieved in a given environment where sufficiently strong eigenmodes cannot be established to support the multiplexed spatial sub-channels. Consequently, there is a danger that such a system may perform no better or potentially worse than a single antenna system. Thus the considerable investment in a multi-antenna system may not provide any return.